How can we store more energy from the sun and the wind?
It could be a scene from a science fiction movie.
Deep in the Nevada desert, thousands of mirrors arrayed in concentric circles face the sky, lit up by the sun.
All this reflected sunshine is directed to the top of a 640 ft (195m) tower standing in their midst.
It's an innovative power plant generating electricity, but not in a way you might expect. And it can carry on doing so even after the sun goes down.
The concentrated light heats up liquid salt pumped to the top of the tower - the temperature reaches 566C (1,050F) - and this heat is then used to make steam to power an electricity generator in another part of the plant.
"The issue with solar traditionally is it is an intermittent power source - you can only produce electricity when the sun is shining," explains Kevin Smith, whose company Solar Reserve built the Crescent Dunes plant.
"But because we store the energy as heat, we can reliably produce electricity 24 hours a day, just like a conventional gas fired power station."
The plant is one of a raft of sustainable energy storage solutions trying to address renewable energy's Achilles heel: its variability - ignoring tidal power's constancy.
If we could store the electricity that sun and wind produce, we could tap into those stores when production dips.
And this is the only way we're going to mount an effective challenge to the dominance of fossil fuels, experts say. Yes, tidal power is renewable and constant, but as yet, we haven't found commercially viable ways of tapping into it.
Storage methods currently being used around the world include batteries, flywheels, geothermal plants, compressed air and hydrogen - even ice. But the the most popular method is hydro power - water pumped to the top of a mountain and then released to power turbines at the bottom.
According to research company Navigant, global energy storage installations are going to rise from about 1,750 megawatts (MW) in 2016 to nearly 11,000 MW by 2020.
"We are moving away from very large conventional power stations that produce electricity which is sent through the grid to consumers, to a system where power is produced in a much more distributed way," says IHS's Sam Wilkinson.
"So storage is obviously a major focus, because it allows you to compensate for and correct for a lot of that fluctuating generation that comes from renewables."
Power to the people
Our homes are increasingly being used for energy storage as well.
Tesla's Powerwall, announced earlier this year, is just one of a crop of new storage batteries designed for domestic use.
The US electric car manufacturer points out that the average household uses more electricity in the morning and evening than during the day, when solar energy is plentiful. So its battery charges during the day from solar roof panels, then powers the home in the evening.
"Without a home battery, excess solar energy is often sold to the power company and purchased back in the evening [at a higher price]," Tesla argues.
"The mismatch adds demand on power plants and increases carbon emissions," it adds.
The same principle is being applied at a new housing development in Hoog Dalem, the Netherlands - part of the Universal Smart Energy Framework project being rolled out by a consortium of companies, including ABB, IBM and Stedin.
Solar-panelled homes are equipped with batteries to store the energy produced during the day for use when the sun goes down.
Used in conjunction with smart meters - which help businesses and domestic users manage electricity use more efficiently - home batteries could revolutionise the way we consume energy, proponents argue.
'Three tennis courts'
Larger battery plants such as Smarter Network Storage (SNS) in Leighton Buzzard, UK, are also likely to play a big role.
The plant comprises 50,000 lithium-ion battery cells, across a site the size of three tennis courts. It can store enough energy to power 1,100 typical UK homes for a day during times of average demand.
Nick Heyward, who manages the UK Power Networks project, explains that our electricity grids need to balance supply and demand at all times, but they are struggling to cope as more wind and solar power comes online.
"One solution could be to add capacity to the existing grid, but that could be very expensive and disruptive," he says. "And then you have the problem of energy curtailment - where wind and solar plants are switched off when there isn't much demand, which is a waste."
Storage systems like SNS offer the ability to export energy to the grid at times of high demand, and keep it in reserve when demand is low.
Mr Heyward believes that if such plants were replicated across the UK, it could unlock more than £600m ($895m) of benefits annually by 2040.
Green but unclean?
But environmentalists are concerned that many of the constituents in the conventional battery, such as cobalt and nickel, are pollutants. So the race is on to find cleaner alternatives.
Aquion's saltwater and manganese oxide battery is made from more sustainable materials than the typical lithium-ion battery, tolerates more charge and discharge cycles, costs less, and doesn't catch fire, the company maintains.
Ambri, meanwhile, has developed a liquid metal battery, which it says is low-cost and emissions free. It also says its liquid electrodes are stronger than the solid ones found in common batteries, and thus less susceptible to failure.
While such innovations may be welcome, the fact remains that the US still derives the vast majority of its electrical storage from pumped hydro - which is not one of the most efficient energy storage technologies.
The number of projects in the pipeline bodes well, says the Energy Storage Association, but we are still a long way from the storage levels we need.
One big impediment is regulation - or lack of it. Governments around the world are only now beginning to respond to this new energy landscape.